1. Field of the Invention
This invention pertains generally to a method and apparatus for detecting a leak in a container by measuring the level of an electrically conductive fluid contained therein. More specifically, it relates to detecting a leak in an underground petrochemical storage tank by measuring the level of the water in the tank.
2. Description of the Prior Art
For many years there have been concerted efforts in the petrochemical industry to prevent environmental contamination resulting from leaks in underground storage vessels. Underground storage tanks are not only found in refineries and other large facilities, but also in gasoline service stations. Such locations may have as many as three or four such tanks and consequently there are a large number of these tanks dispersed over an incredibly large area.
The governments of the United States and other countries have for some time issued and enforced regulations requiring the detection and correction of leaks in an effort to prevent environmental damage, particularly contamination of underground water supplies. A sizable industry has developed to provide the technology and skills needed to enable service station owners and others to comply with these federal regulations. Compliance typically involves transporting manpower and materials to the site of the tank and performing a series of tests designed to measure certain conditions inside the tank on a regular basis. These conditions yield indications of whether a leak exists and, preferably, the size of the leak. In the United States, the federal government now also regulates and certifies the instruments used in these tests.
One commonly tested condition is the level of water in the tank. A leak that allows a chemical to seep from a tank into the environment also allows water to seep into the tank. Because water is immiscible with many chemicals and fuels, and because water is heavier than gasoline, the two fluids separate into two layers with the water on the bottom. It is therefore possible to position a probe on the bottom of the tank to measure the level of water in the tank. By monitoring the change in the level of the water over a preselected period of time, it is possible to determine whether a leak exists and the size of the leak.
Many of these tests take advantage of the electrical conductivity of the water which leaks into the tank. It is well known that gasoline is a very poor conductor of electricity. It is also well known that pure water is also a poor conductor, but very little water that seeps into a tank is pure. The water in the tank usually has a fairly high degree of ionic activity that renders it electrically conductive. It is therefore possible to detect and quantify the impure water in the tank by measuring the level of this electrically conductive fluid once the water and gasoline have separated into their respective layers, and such methods are known in the art. However, the known methods, and the apparatus used in those methods, are characterized by a number of disadvantages and limitations which limit their capabilities and increase their cost.
An example of such a limitation is that in the United States, federal regulations require instruments for use in testing gasoline storage tanks to detect leaks as small as 0.1 gal/hr. Further, these tanks can have extremely large dimensions. Such large dimensions sometimes result in a very thin layer of water in the tank, having a depth perhaps as small as 0.020 inches. Because of the thin (or shallow) layer of water and the slow rate of change, a depth measuring device must therefore be capable of operating with relatively high resolution and precision. Because of the limited resolution capabilities of known methods for quantifying the influx of water and the long time required for sufficient water to leak into the tank to be able to be detected by these known devices as a result of their limited resolution, known methods may require several hours of testing time per tank. In installations in which there are several tanks, the time required to test each tank causes testing costs to approach the prohibitive level.
Known probes for use in quantifying the water in a tank are comprised of a plurality of alternating conductive and insulating disc or plates which are pressed onto a post extending out of the distal end of the probe, e.g., the end opposite the end which is attached to the electrical cable from which the probe is suspended in the tank. Each conductor is separately wired in parallel into a resistor network for summing the current pulsed into the post, the water completing the circuit to the extent that the water covers a certain number of the conductive plates. It is this construction which limits the resolution of known methods because the plates cannot be made in but a certain minimum thickness to allow them to be handled and to withstand the physical stresses of being mounted on the post. Further, the alternating plates and insulators must be retained on the end of the post, and a relatively thick (compared to the thickness of the plates) retainer, shaped like the plates, is generally used for that purpose. Consequently, even when the distal end of that post is positioned on the floor of the tank being tested (and particulate matter may prevent such positioning), it is not possible to detect a layer of water that is thinner, or shallower, than the thickness of that retainer.
There are many other disadvantages of probes of this construction. For instance, because of the conditions in which the probe is used and the stresses resulting from the above-described construction, the plates, post and probe must be manufactured from stainless steel, a material which is notoriously expensive to mill and fabricate (especially in the minimum thickness of the plates). Further, the plates must be assembled onto the post and then hard wired in separate connections, both of which are time-consuming processes. Also, because of the use of the electrically conductive water to complete a circuit, the salts in the water plate onto the plates during the test, decreasing current flow to the point that the deposits must be cleaned from the plates at regular intervals.
Another disadvantage of such known probes is their length. These probes are generally cylindrical and, in addition to housing the circuitry for quantifying the influx of water into the tank, include the necessary components for sensing temperature (for correcting depth changes for changes in temperature which cause volume changes) and for detecting the bubbles created in the fluid in the influx of air. The latter function results from a leak test which is conducted by evacuating the ullage in the tank and detecting the bubbles formed in the fluid in the tank by the air which enters the tank below the surface of the fluid in accordance with the method described in U.S. Pat. No. 4,462,249, assigned to the owner of the present invention and hereby incorporated herein in its entirety by this specific reference thereto. A hydrophone is mounted in the probe body for detecting the resulting bubbles, but due to the length of the probe body and the post extending from one end of the probe for retaining the plates and insulators, when the distal end of the post rests on the bottom of the tank, the hydrophone is positioned about eighteen inches above the bottom of the tank. This long length also results from the space necessary to individually wire each conductor into the resistor network. Of course, the hydrophone must be submerged to function properly, and the size requirements resulting from this construction are such that the tank must contain a minimum of about eighteen inches of fluid to insure that the hydrophone is submerged. There are many circumstances in which that minimum depth requirement causes problems. For instance, when a testing crew arrives on site and discovers that a gasoline service station storage tank does not include that minimum depth of product, the station operator must buy more product. Additional product takes time to procure and costs extra because of the unscheduled nature of the delivery. Another alternative which is also costly is that the testing crew must return at another time.
It is, therefore, a principal object of the present invention to provide a method and apparatus capable of overcoming these disadvantages and limitations of prior known methods and apparatus for detecting leaks in storage tanks.
It is also an object of this invention to provide a method and apparatus that measures the level of an electrically conductive fluid in a vessel, and in particular, in a vessel, containing immiscible fluids with a high degree of resolution.
It is also a feature of this invention that such an apparatus be sufficiently small and lightweight so that it can be easily transported to different varying locations.
It is a further feature of this invention that the apparatus be compatible with existing technology in the industry.
It is still a further feature of this invention that the apparatus complies with current federal regulations regarding operation and performance of these types of instruments and that, if properly used, it will enable gasoline storage tank operators and owners to also comply with federal regulations.